Among other devices, sensors which ascertain the rotational wheel speed are used to measure the speed of a rail-bound vehicle. However, under certain operating conditions such as in uphill driving, in acceleration or braking operations or under certain weather conditions, this measuring method may turn out to be unreliable for ascertaining the driving speed or the absolute speed of the rail-bound vehicle. This is why modern rail-bound vehicles also utilize sensors that use the reflection of optical beams or acoustic signals at the undersurface for determining the absolute speed of the vehicle. In this measuring method as well, the reliability may be subject to considerable fluctuations due to external influences such as soiling of the sensors or weather-related reduced reflectivity of the undersurface.